1. Field of the Invention:
The present invention relates to a high-frequency semiconductor device and in particular, to a thick film hybrid integrated circuit for power amplification in a radio communication apparatus or the like which employs a high-frequency transistor.
2. Description of the Related Art:
Hybrid integrated circuit apparatus for power amplifying modules in a radio communication apparatus have been proposed recently. The overall size of such hybrid integrated circuit apparatus has been reduced. The semiconductor device of this type will be described below with reference to the accompanying drawings.
FIG. 1A is a schematic plan view of a semiconductor element portion of a conventional, high-frequency semiconductor device, e.g., a thick-film hybrid integrated circuit. FIG. 1B is a sectional view taken along the line B--B of FIG. 1A, FIG. 1C is a sectional view taken along the line C--C of FIG. 1A, and FIG. 1D is a sectional view taken along the line D--D of FIG. 1A. In these figures, a conductive film 2 for input, a conductive film 3 for output, and a conductive film 4 for grounding are formed by metallization of an insulating substrate 1 which is formed of a ceramic material, such as alumina. The input film 2 is connected to a semiconductor element 7, e.g., npn type transistor, by a wire 5a through a bonding pad 6. The conductive grounding film 4 is connected to the semiconductor element 7 by a wire 5b through a bonding pad 8.
The semiconductor element 7 emits a large amount of heat because it is used for high-power amplification. Thus, a heat radiating plate 9 is disposed on the conductive output film 3 to improve the radiation of heat from the semiconductor element 7. The semiconductor element 7 is fixed on the heat radiating plate 9 by brazing or the like. A MOS (metal oxide semiconductor) capacitive element 10 is disposed on the conductive grounding film 4 owing to an extremely low impedance of the high-frequency transistor. On the MOS capacitive element 10 are formed a pad 11a for the MOS capacitive element 10 and a pad 11b whose bottom surface is grounded.
The bonding pad 8 for grounding and the pad 11b are connected by the wire 5b. If the wires 5a and 5b are long, high performance cannot be obtained in the high frequency area due to the influence of a reactance. Hence, in order to reduce the reactance, the wires 5a and 5b are connected in parallel, and a grounded conductive bridge 12 including bridging portion 12b and legs 12c and 12d and for to which wire 5b is connected is provided. Also, a heat radiating plate 13 formed of, for example, copper for improving the radiation of heat from the semiconductor element 7 is disposed on the rear surface of the insulating substrate 1.
In the high-frequency semiconductor apparatus arranged in the above-described manner, since the wires 5a and 5b are shortened to achieve improvement in the high-frequency characteristics, the semiconductor element 7 is fixed to the heat-radiating plate 9 by means of die bonding or the like. It is known that heat is transmitted at an expanding angle of about 45 degrees. The heat-radiating plate 9 therefore radiates heat from the semiconductor element 7 only in one direction. Thus, in order to achieve a reduction in the heat resistance of the semiconductor element 7 and of the heat-radiating plate 13, the area at which the heat-radiating plate 9 is in contact with the insulating substrate 1 must be increased. However, an increase in the size of the heat-radiating plate 9 makes reduction of the size of the insulating substrate 1 difficult. Furthermore, it is desirable that the bonding pads for the wires 5a and 5b be formed in the same plane increase the strength of adhesion.
In the above-described high-frequency semiconductor device, the area at which the semiconductor element 7 is in contact with the insulating substrate 1 cannot be increased due to the presence of the MOS capacitive element 10. Therefore, the heat resistance of the semiconductor element 7 and of the heat-radiating plate 13 cannot be reduced, thereby making it impossible to reduce the size of the insulating substrate 1. Furthermore, the bonding pads for the pads 11a and 11b formed on the MOS capacitive element 10, the bonding pads 6 and 8 on the semiconductor element 7, and a bonding pad 12a for the conductive bridge 12 for grounding are at different heights, reducing the reliability of the wire bonding.